Measurement of hydrocarbon fuel concentration by 3.39 μm He-Ne laser infrared absorption technique

The 3.39 μm He-Ne infrared laser absorption technique is useful for measuring hydrocarbon concentration. To measure the concentration of propane or methane in hydrocarbon fuels, molar absorption coefficients for 3.39 μm wavelength were investigated in the temperature range 285 to approximately 420 K and in the pressure range 100 to approximately 800 kPa. It was found that (1) the absorption coefficient ε is independent of temperature, and (2) the pressure has little effect on ε in propane, but a strong effect on ε in methane. By means of this technique, the fuel concentration in the vicinity of a spark plug and HC concentration in an exhaust pipe of a spark ignition engine were measured under the condition of no residual gas and homogeneous mixture. For the lean mixture operating condition, although cycle-to-cycle fluctuation of the fuel concentration was very small, the fluctuation of pressure in the cylinder was large.